Mice deficient in glycerol-3-phosphate acyltransferase-1 have a reduced susceptibility to liver cancer

Abstract

The risk of hepatocellular carcinoma increases with the persistence of non-alcoholic fatty liver disease. Triacylglycerol synthesis is initiated by glycerol-3-phosphate acyltransferase (GPAT). Of four isoforms, GPAT1 contributes 30-50% of total liver GPAT activity, and we hypothesized that it might influence liver susceptibility to tumorigenesis. C57Bl/6 mice deficient in GPAT1 were backcrossed 6 times to C3H mice. After exposure to the carcinogen diethylnitrosamine (DEN) and the tumor promoter phenobarbital, male Gpat1⁻/⁻ mice, compared with controls (Gpat1⁺/⁺), had 93% fewer macroscopically visible nodules per liver at 21 weeks of age and 39% fewer at 34 weeks of age. Microscopically, control mice had increased numbers of foci of altered hepatocytes, particularly the basophilic subtype, as well as more, and malignant, liver neoplasms than did the Gpat1⁻/⁻ mice. At 21 weeks of age, 50% (4/8) of control mice (50%) had hepatocellular adenomas with an average multiplicity (tumors per tumor-bearing-animal) of 4.3, while none occurred in 8 Gpat1⁻/⁻ mice. At 34 weeks of age, all 15 control mice (100%) had hepatocellular adenomas with an average multiplicity of 5.2 compared to an incidence of 93% in Gpat1⁻/⁻ mice and multiplicity of 3.1. HCCs were observed in 13% of control mice and in only 6% of Gpat1⁻/⁻ mice. These data show that alterations in the formation of complex lipids catalyzed by Gpat1 reduce susceptibility to DEN-induced liver tumorigenesis.

FIGURE 1

Gpat1^−/− mice on the C3H strain background had decreased Gpat1 expression and activity in liver. (A) Gpat1 mRNA abundance in C57Bl/6 and C3H control livers. (B) Gpat2, Gpat3, and Gpat4 mRNA abundance in control and Gpat1^−/− livers on the C3H background. (C) GPAT-specific activity in control and Gpat1^−/− livers on the C3H background. Data represent the average ± SEM; *p ≤ .05, **p ≤ .01, ***p ≤ .001, between genotypes.

FIGURE 2

Gpat1^−/− mice had reduced susceptibility to liver cancer. (A) Average number of macroscopically visible liver nodules in control and Gpat1^−/− 21-week-old mice (control, n = 12; Gpat1^−/−, n = 11) and 34-week-old mice (control, n = 15; Gpat1^−/−, n = 19). (B) Average of macroscopically visible nodules per liver lobe at 34 wk of age (n = 15). Data represent the average ± SEM; *p ≤ .05, **p ≤ .01, between genotypes.

FIGURE 3

Gpat1^−/− livers from 8-week-old, non-DEN-exposed mice had increased rates of hepatocyte proliferation. Percentage of hepatocytes positive for BrdU incorporation after a 48-h exposure.

FIGURE 4

Gpat1^−/− and control hepatocytes increased BrdU incorporation similarly in response to Wy-14643. (A) Liver weight, as a percentage of body weight; and (B) BrdU incorporation into hepatocytes in control and Gpat1^−/− mice with or without (No Trt) 1 wk of exposure to Wy-14643, n = 3. Data represent the average ± SEM; #, p ≤ .05 between no treatment and Wy-14643–treated mice within the genotype.

FIGURE 5

Representative photomicrographs of lesions in C3H livers. (A) Hematoxylin and eosin-stained basophilic focus of altered hepatocytes; (B) clear cell focus of altered hepatocytes; (C) low magnification of an hepatocellular adenoma; (D) high magnification of an hepatocellular carcinoma characterized by pleomorphic hepatocytes forming irregular trabeculae that are often three cells or greater in width.